It is well known that the epoxidation of olefinic compounds with hydrogen peroxide may be effectively catalyzed by certain synthetic zeolites containing titanium atoms (see, for example, U.S. Pat. No. 4,833,260). While selectivity to the desired epoxide is generally high, U.S. Pat. No. 4,824,976 proposes that the non-selective ring-opening reactions which take place when epoxidation is performed in a protic medium such as water or alcohol may be suppressed by treating the catalyst prior to the reaction or during the reaction with a suitable acid neutralizing agent. The neutralizing agent is said to neutralize acid groups on the catalyst surface which tend to promote by-product formation. Neutralization, according to the patent, may be accomplished with water soluble basic substances chosen from among strong bases such as NaOH and KOH and weak bases such as NH.sub.4 OH, Na.sub.2 CO.sub.3, NaHCO.sub.3, Na.sub.2 HPO, and analogous potassium and lithium salts including K.sub.2 CO.sub.3, Li.sub.2 CO.sub.3, KHCO.sub.3, LiHCO.sub.3, and K.sub.2 HPO.sub.4, alkali and/or alkaline earth salts of carboxylic acids having from 1 to 10 carbon atoms and alkali and/or alkaline earth alcoholates having from 1 to 10 carbon atoms.
Co-pending U.S. application Ser. No. 08/396,319, filed Feb. 28, 1995, discloses that by carrying out a titanium silicalite-catalyzed epoxidation in the presence of low concentrations of a non-basic salt (i.e., a neutral or acidic salt) selectivity to epoxide may unexpectedly be significantly improved by reducing the quantity of ring-opened by-products formed.
We have now found that while epoxide ring-opening may be effectively suppressed by performing the epoxidation in the presence of a suitable source of ammonium, alkali metal, or alkaline earth metal cations, whether basic, neutral, or acidic in character, non-selective hydrogen peroxide decomposition to oxygen and water tends to gradually increase as the titanium silicalite catalyst ages. For example, when titanium silicalite is used in a continuous fixed bed system to epoxidize propylene in the presence of a cation source such as ammonium hydroxide, selectivity to the desired propylene oxide product decreases over time while selectivity to oxygen increases to the range of about 8 to 15%. The mechanism responsible for this loss in epoxide selectivity is not well understood. It would be highly desirable, however, to find a means of alleviating the effects of aging on catalyst performance such that epoxide ring-opening and hydrogen peroxide decomposition are simultaneously suppressed in order to maximize the yield of epoxide obtained over the life of a particular catalyst charge.